In the manufacture of paper from wood, among the first steps are debarking the logs and conversion of the wood logs into chips which are subsequently cooked (digested) to commence the separation of the wood fibers from other components of the chips.
In a common sequence of operation, the logs are loaded with a crane from a storage yard to a debarking drum where at least a major portion of the bark is removed from each log and thereafter the debarked logs are discharged onto the conveying system to a chipper. This conveying operation to the chipper is commonly designed to spread the logs into loose bundles of two or three parallel aligned logs. These bundles are loaded onto a conveyor, one bundle behind another bundle, so that the bundles are fed consecutively to the chipper, thereby ensuring a smooth and continuous flow of logs into the chipper such that the chipper is efficiently utilized but does not become clogged. A clogged chipper commonly is shut down when a clog develops, with attendant loss of operating time and the cost of maintenance required to clear the clog.
Usually two chain conveyors, or occasionally belt conveyors, are employed in conveying the bundles of logs to a chipper. These two conveyors are directionally aligned with one another, the upstream end of the downstream conveyor terminating short of the downstream end of the upstream conveyor, thereby developing an open gap between these adjacent conveyor ends. The use of two conveyors permits, among other things, the development of a gap between the contiguous ends of the conveyors through which trash, such as pieces of bark, too small pieces of wood, and/or other undesirable materials to fall by gravity through the gap and be collected or transported to a remote site, to be used as fuel, for example.
In a log conveying system, a gap between conveyor ends may range from about 3 inches to about 10 inches in length (measured in the direction of movement of the conveyors) and will extend fully across the width of the conveyors.
As noted, the gap between the conveyor ends is desirable from the standpoint of allowing debris, mainly pieces of bark and occasionally some wood pieces, to be separated from the logs as they are moved to the chipper. However, the open gap between the adjacent ends of the two conveyors permits chunks of useable wood or even small or short logs to fall into the gap, rather than being transferred from the downstream to the upstream conveyor. These small or short logs in the gap tend to cause other debris to collect within the gap, resulting in the development of a clog within the gap. Such clogs commonly develop into a size and degree of compaction within the gap as to either overload one of both of the conveyor motors, stopping the conveyor(s) or to physically damage one or both of the conveyors, especially when employing the commonly used chain conveyors. Even if the clog does not halt the movement of the conveyors, it disrupts the desired even flow of logs to the chipper and/or can even cause dislodgement of one or more logs from the conveyor, among other possible disastrous results.
In certain mills which process smaller diameter logs, the loss of useful wood via the gap is exasperated, thus creating a problem in accommodating the desired functioning of the gap as a remover of debris while permitting usable wood pieces to be conveyed across the gap and into the chipper. Mere adjustment of the width of the gap between the conveyor ends is not feasible in that such requires movement of the conveyors toward or away from one another, a major time-consuming and costly operation.
Placing a shield within the gap to lessen the width of the gap also is unworkable in that mere reduction of the width of the gap increases the likelihood of a clog developing. One approach to this problem has been to install a fast driven roller within the gap with the intent that relatively larger pieces of wood will be propelled across the gap, while bark and smaller pieces of wood will merely fall through the gap. In one embodiment, this roller is mounted for vertical movement within the gap, but not permitted to move different lateral distances at each of its opposite ends. Thus, a piece of wood, for example, which is larger that the fixed open space defined between the roller and a conveyor end, can clog the gap. That is, there is no flexibility of lateral movement of the fixed roller, only up and down movement of the roller. In certain mills, the character of the bark debris changes with the season, for example, longer (stringy) bark strings may be present when processing Spring wood. During such times, it is desirable to have a wider gap to permit such debris to pass through the gap, rather than being conveyed to the chipper and without clogging developing within the gap.